Digital circuits in electronic systems such as computer processors, handheld electronic devices and digital cameras may include N-channel field-effect transistors (NFETs), and P-channel field-effect transistors (PFETs) arranged in a complimentary fashion to perform logical functions. The threshold voltage (VT) of a field-effect transistor (FET) is a characteristic that may be related to the electrical performance of circuits that include NFET and PFET devices. The threshold voltage of a FET device may change over time, as a result of transistor aging mechanisms, such as negative-bias temperature instability (NBTI), positive-bias temperature instability (PBTI) and hot-carrier injection.
A general trend of decreasing FET device geometries, such as gate length, may make smaller FETs increasingly vulnerable to aging mechanisms. Changes in FET threshold voltages resulting from aging mechanisms may result in decreased operating speed and/or drive strength of a digital circuit, such as a logic, repower, or memory circuit that includes NFETs and PFETs.